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iv is t-Drill Grinder.

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197

wheel, and fluting, &c., on the small wheel. It is, „,made, by
Messrs. Selig, Sonnenthal & Co., and a small attachment is
provided to carry the wheel when grinding milling cutters, the
latter being then held on the spindle of the machine, and the

wheel driven by catgut band Emery wheels may be used for
general grinding and removing of surplus material and thereby
save a large amount of fitting. (See p. 1030.)

Twist-Drill Grinder.—These are of various designs,the one
in Fig. 213 being made by Messrs. Selig, Sonnenthal & Co. The end
of a twist-drill would be conical in shape but for the clearance or
relief angle. The true surface becomes, accounting for clearance,
a cone having a helix for its base, and enclosing an angle of. 118°.
A section of this cone, then, made at right angles to one of the
slant sides, would give a curve deviating slightly from a hyperbola,
due to the clearance. We will now examine the method by which
this hyperbolic surface is ground in Messrs, Selig's machine.